How to un-extrude an object?
It seems you have extruded several times but did not moved the extruded vertices.

In this particular case, to remove that:


Select the top extruded face (CtrlTab then choose 'face' and right click to select the face)
Use CtrlNumpad+ to extend the selection. Do that several times. You know you are ok when the selection come to the surrounding parts, when it is deselect twice using CtrlNumpad-
X to delete the selected vertices
CtrlTab then choose 'edge' this time
AltRMB to select the edges surrounding the hole
F to remake the face